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The families of flowering plants

L. Watson and M.J. Dallwitz

Salicaceae Mirbel

Excluding here numerous genera formerly referred to Flacourtiaceae (q.v.).

Habit and leaf form. Trees and shrubs (including some procumbent and ‘almost herbaceous’ Salix species); leptocaul, or pachycaul (e.g., Salix alaxensis, S. richardsonii). Helophytic (often on riversides, in fens and bogs, dune slacks, etc.), or mesophytic. Heterophyllous (in some Populus species), or not heterophyllous. Leaves deciduous; small to large; alternate (mostly), or alternate to opposite; spiral, or spiral to distichous; flat; petiolate (the distal end of the petiole characterised by one or more closed rings of xylem and phloem, which in Populus are often vertically superimposed); non-sheathing; simple; epulvinate. Lamina entire (mainly), or dissected (in some Populus species); when dissected, somewhat pinnatifid, or palmatifid (e.g., in summer leaf blades and on suckers of P. alba); pinnately veined (mostly), or parallel-veined (some arctic species being campylodromous or parallelodromous); cross-venulate. Leaves more or less exstipulate (e.g., Chosenia, some Salix species), or stipulate (usually, at least on vigorous shoots). Stipules intrapetiolar (lateral to the petioles); free of one another; caducous (often), or persistent. Lamina margins entire (to undulate or crispate), or dentate (occasionally, more or less, or shallowly lobed), or crenate (or crenulate), or serrate (or serrulate, or spinulose-serrulate, the margins sometimes furnished with vesicular glands). Vegetative buds scaly. Leaf development not ‘graminaceous’. Domatia occurring in the family (seen in Populus); manifested as hair tufts.

Leaf anatomy. The leaf lamina dorsiventral (usually), or bifacial (e.g., isobilateral recorded species of both genera). Mucilaginous epidermis present (commonly), or absent. Stomata present; on both surfaces (frequently), or mainly confined to one surface (then abaxial); often paracytic (sometimes with two subsidiaries at either side). Hairs usually (?) present; eglandular and glandular (the former simple and unicellular, sometimes thick-walled basally; vesicular glands lined with radially arranged secretory cells recorded mainly in Salix species). Adaxial hypodermis present, or absent. The mesophyll containing crystals. The crystals druses and solitary-prismatic. Minor leaf veins without phloem transfer cells (Salix).

Axial (stem, wood) anatomy. Young stems cylindrical. Secretory cavities absent. Cork cambium present; initially superficial. Nodes tri-lacunar. Primary vascular tissues in a cylinder, without separate bundles; collateral. Internal phloem absent. Cortical bundles absent. Medullary bundles absent. Secondary thickening developing from a conventional cambial ring. Primary medullary rays narrow.

The wood ring porous to diffuse porous. The vessels fairly small. The vessel end-walls simple. The vessels without vestured pits; without spiral thickening. The axial xylem without fibre tracheids; with libriform fibres; including septate fibres, or without septate fibres. The fibres without spiral thickening. The parenchyma apotracheal (exclusively terminal). The secondary phloem commonly containing crystalliferous fibres, stratified into hard (fibrous) and soft (parenchymatous) zones, or not stratified. ‘Included’ phloem absent. The wood not storied.

Reproductive type, pollination. Plants dioecious. Pollination variously anemophilous, or entomophilous.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in catkins (these pendulous in Populus, usually erect in Salix). The ultimate inflorescence units (i.e., the catkin) racemose. Inflorescences terminal (and appearing after the leaves, e.g. in Salix herbacea, S. reticulata), or axillary (usually, then lateral on the previous year’s wood, and maturing with or more commonly before the leaves); catkins, the males and females on different plants; sometimes somewhat pseudanthial, or not pseudanthial. Flowers individually bracteate (with each member of the catkin subtended by a bract, which is generally entire in Salix, and lobed or cut in Populus); ebracteolate (according to the usual interpretations of catkin structures); minute to small. Free hypanthium absent.

Perianth absent, or vestigial (a supposed vestigial calyx being represented in Populus by a cupular disk, or in Salix by one or two to several small, sometimes fringed, often unequal and sometimes united nectariferous scales).

Androecium 1, or 2 (most Salix species have two while some (e.g., S. sitchensis) genuinely have only one, and others such as S. purpurea have a pair of coalesced filaments but two anthers), or 3–30(–60) (with Populus usually having ‘several to many’). Androecial members branched (?), or unbranched (‘additional’ stamens in Salix having been variously interpreted as representing dedoublement of the medians or ‘development of the inner series’); free of one another, or coherent (by coalescence of filaments); when coherent, 1 adelphous. Androecium exclusively of fertile stamens. Stamens (1–)2–30(–60). Filaments not appendiculate. Anthers basifixed; dehiscing via longitudinal slits; tetrasporangiate. Endothecium developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or isobilateral, or decussate. Anther wall initially with one middle layer, or initially with more than one middle layer; of the ‘monocot’ type. Tapetum glandular. Pollen grains aperturate, or nonaperturate (Populus); when aperturate, (2–)3(–6) aperturate; colporate (colporoidate); 2-celled.

Gynoecium 2(–4) carpelled. The pistil 1 celled. Gynoecium syncarpous; synovarious, or synovarious to synstylovarious (with styles partially fused), or synstylovarious; superior. Ovary 1 locular. Gynoecium when 2-carpelled (i.e. usually), transverse. Stigmas 2–4; dorsal to the carpels, or commissural. Placentation basal, or parietal. Ovules in the single cavity 4–40(–50) (generally described as ‘(2-) several to many’, but most Salix species have fewer than 20 per carpel, some have ‘few’, and some Salix species and Chosenia arbutifolia consistently exhibit only 2 ovules per carpel); funicled; ascending; arillate (the aril becoming silky-hairy and surrounding the seed); anatropous; unitegmic, or bitegmic (when the inner integument is weakly developed); crassinucellate. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids with filiform apparatus. Endosperm formation nuclear. Embryogeny onagrad, or asterad.

Fruit non-fleshy; dehiscent; a capsule. Capsules 2–4 valvular. Seeds scantily endospermic, or non-endospermic. Endosperm when present, oily. Cotyledons 2; plano-convex. Embryo chlorophyllous (2/8); straight.

Seedling. Germination phanerocotylar.

Physiology, phytochemistry. C3. C3 physiology recorded directly in Populus, Salix. Anatomy non-C4 type (Populus, Salix). Sugars transported as sucrose, or as oligosaccharides + sucrose, or as sugar alcohols + oligosaccharides + sucrose (but sucrose always predominating, in 12 Populus and Salix species). Cyanogenic (?), or not cyanogenic. Alkaloids absent (usually), or present. Arbutin absent. Iridoids not detected. Saponins/sapogenins absent. Proanthocyanidins present, or absent; when present, cyanidin, or cyanidin and delphinidin. Flavonols present; kaempferol and quercetin. Ellagic acid absent (2 genera, 8 species). Aluminium accumulation not found. Sieve-tube plastids S-type.

Geography, cytology. Frigid zone and temperate. Absent from Australasia and New Guinea, otherwise cosmopolitan, but mostly temperate Northern hemisphere. X = 11, 12, 19.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Violiflorae; Salicales. Cronquist’s Subclass Dilleniidae; Salicales. APG III core angiosperms; core eudicot; Superorder Rosanae; fabid. APG IV Order Malpighiales.

Species 350. Genera 4; Salix, Populus, Chosenia.

General remarks. This description corrected and extended via comments from George Argus (1998). Rendle (1959) discusses ‘classical’ interpretations of catkin and floral morphology.

Economic uses, etc. Many are cultivated as ornamental trees and shrubs, and Salix contributes timber (notably for cricket bats) and withy twigs for basketry. Willows are widely planted for erosion control, land reclamation and as biomass for energy, and their roots are prone to destroy the foundations of houses and drains.


There is a willow grows ascaunt the brook,
That shows his hoar leaves in the glassy stream
(‘Hamlet’, iv., 7)

Tell him, in hope he’ll prove a widower shortly,
I’ll wear a willow garland for his sake
(‘3rd King Henry the Sixth’, iii., 3)

O, had the monster seen those lily hands
Tremble, like aspen leaves upon the lute
(‘Titus Andronicus’,ii., 5)

Illustrations. • Le Maout and Decaisne: Salix, Populus. • Salix safsaf: Thonner. • Salix alba: Hutchinson. • Populus alba, P. canescens, P. nigra and P. tremula: Eng. Bot. 1299–1302, 1868. • Populus tremula (B. Ent.). • Salix caprea, S. cinerea and S. triandra: Eng. Bot. 1313, 1327 and 1313 (1868). • Salix fragilis, S. pentandra and S. viminalis: Eng. Bot. 1306, 1303 and 1322 (1868). • Salix repens, S. aurita and S. ambigua (= aurita x repens): Eng. Bot. 1356, 1330 and 1355 (1868). • Salix lanata, S. phylicifolia and S. reticulata: Eng. Bot. 1367, 1336 and 1379 (1868). • Salix sp., male (B. Ent.).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 5th March 2018.’.